After experimenting with a scene that had a ton of it I can say with certainty that it can be caused by a lot of things and if you are in that situation you have to try all of the below and maybe go backwards and find whats causing yours (outdoor scene):
-
remove all post processing effects that could cause it: no sharpness, no bloom, no chromatic aberration, no high contrast from pp effects
-
put a base material on everything with mid values (0.5 for everything except metallic zero) and see if its all gone, this is just for ruling out if its a lighting issue or material issue (or both)
-
if there are still a ton of it what might sound stupid but it actually made my scene way worse as I was already using real world values and tried the default “10” for the sun and got 10x more white pixels - set the sun light to 100k, sky light above 1 maybe to 3 (if its a hdr wrapped skylight - 20k int for the image), and adjust the post processing volume exposure to get a similar visual result to your original scene setup. real world values seem to work better for path tracer.
-
if the white pixels appear along edges of certain objects it is very likely some material setting that causes most of it. I’ve set everything between 0.1 an 0.9 and got rid of at least half of the white pixels
-
path tracer does not like high frequency textures either so a busy normal map or any other map can also cause problems
-
what I can’t fully grasp yet is white pixels where two different materials meet with all of the above already removed about 85% of the fireflies I still could not get rid of these yet. sometimes even on one material where shadow and light meets. could be a shadow type problem.
The look of the scene will definitely change slightly, wont be able to get a perfect match to the original, but you can always adjust it in post to try to match it better with the rest of the footage if needed.
What I’ve also seen in my case is that there are a lot more hot pixels in areas where the camera is not focusing (turned off DOF to see the scene better) → A slight DOF will hide most of it, which is actually not a bad idea as most cameras will have at least a minimal dof in most cases (if its not a still scene shot from a tripod with long exposure)
in the end ive managed to remove around 95-98 percent of the white pixels by also increasing the overall brightness of the scene a bit with post processing. no idea why that helped as it was already well lit.